There are a few significant differences between formalin and formaldehyde. 

Formaldehyde

Formaldehyde is a basic chemical compound that has only one component. It is an aldehyde that occurs as a colorless gas at room temperatures. 

Formaldehyde is an essential substance used extensively as a common precursor for more complex materials such as phenol formaldehyde resin, melamine resin, and urea formaldehyde resin among others. 

Formalin

Formalin is a formulation of formaldehyde in aqueous solution and is composed of three components – formaldehyde, methanol, and water. It occurs as a colorless liquid at room temperature. 

Formalin is widely used as a disinfectant and germicide as well as for preventing cell decay and preserving tissue samples.